We have purified a human lung tumor-associated antigen (hLTAA) to apparent homogeneity from an extract of a primary small cell carcinoma. The antigen, which was originally detected by double diffusion analysis in 84/98 lung tumor extracts, but not in normal tissues or other tumors, is a protein which exhibits size and charge heterogeneity. The protein has been characterized in terms of its sedimentation and diffusion coefficients, subunit size, isoelectric point, immunochemical reactivity with various antisera, and was shown to be distinct from alpha1-antichymotrypsin, a normal protein which cross-reacts with another lung tumor-associated antigen we have studied. A radioimmunoassay (RIA) has been developed despite considerable difficulties encountered in radioiodinating the antigen. This RIA clearly distinguishes normal from lung cancer sera. The antigen is being purified from a human lung cell line which will be used as a constant source of antigen. A similar reactivity was also identified in the urine of certain patients with malignancies. Monoclonal and polyclonal antibodies are being produced against purified hLTAA. An assay for mouse MAF has been developed which measures plasminogen-dependent fibrinolysis and will be used in the purification and characterization of this lymphokine.